All ICs have internal substrate diodes that turn on whenever any pin is pulled to a polarity opposite it's normal polarity. When the internal diodes turn on it usually latches up the circuit and it blows up from large current. The external diodes turn on at a lower voltage than the internal ones and prevent the latch up.
I agree.
(D1) If the input is shorted out, after running for a while, power from the output capacitor will flow backwards through the regulator into the short. The regulator can handle this if the output cap is small. With large output caps the regulator will die. The input to output diode protects against input shorts.
You mean, without D2, C2 connected to ADJ would destroy the LM? But, there isn't C2 on the original schematics (without short circuit protection), so the question arises - what is the purpose of C2 :-/
So, without the protection diodes, input shorcut would cause only no output voltage and nothing else? But the note under the schematics says: D1 protexts the device against input shortcut - why protection if the only consequence would be no output voltage?
All ICs have internal substrate diodes that turn on whenever any pin is pulled to a polarity opposite it's normal polarity. When the internal diodes turn on it usually latches up the circuit and it blows up from large current. The external diodes turn on at a lower voltage than the internal ones and prevent the latch up.
I agree.
(D1) If the input is shorted out, after running for a while, power from the output capacitor will flow backwards through the regulator into the short. The regulator can handle this if the output cap is small. With large output caps the regulator will die. The input to output diode protects against input shorts.
You mean, if C2 is removed - there is no need for D2? So why would we put C2 in the first place? There are only C1 and C3 on the original schematich (without protection diodes).?
I think you are looking at the datasheet from ST Micro who are Italian. Their text does not explain much in English.
National Semi invented the LM317 so you should look at their datasheet where everything is explained in English with details.
National Semi explains the function of C2 and the functions of the diodes on their dsatasheet.
I think you are looking at the datasheet from ST Micro who are Italian. Their text does not explain much in English.
National Semi invented the LM317 so you should look at their datasheet where everything is explained in English with details.
National Semi explains the function of C2 and the functions of the diodes on their dsatasheet.
Arrghh ))) Thank you very much! The schematics from my first post was in fact from the SGS Thompson datasheet but in the same folder I have the following LM datasheets:
LM317 - Fairchild
LM317 - National Semiconductor
LM317 - ON Semiconductor
LM317k - SGS Thompson
LM317L - ST Microelectronics
and I have even marked the National Semiconductor's datasheet as the best but have overlooked the explanation!!
So, without the protection diodes, input shorcut would cause only no output voltage and nothing else? But the note under the schematics says: D1 protexts the device against input shortcut - why protection if the only consequence would be no output voltage?
The note says: D2 protects against output shortcut - not D1 :-/
When you short the input to ground, the output cap holds a voltage for a while so the input-to-output voltage is reversed with the output being higher than the input: the substrate diode (inherent in the power transistor) turns on and the output cap discharges into the chip which can cause death.
Exact same thing for the cap on the adjust pin. It holds the voltage up as the IC voltages come down, internal diode turns on and cap discharges. Peak discharge current damages IC.
Protect diodes provide parallel path for capacitors to discharge through.
I think you are looking at the datasheet from ST Micro who are Italian. Their text does not explain much in English.
National Semi invented the LM317 so you should look at their datasheet where everything is explained in English with details.
National Semi explains the function of C2 and the functions of the diodes on their dsatasheet.
DO NOT operate an LM317 at its maximum allowed temperature. If it reaches such a high temperatrure many times then the chip might fracture from "thermal fatigue". It expands when hot and shrinks when cooled over and over.